The City of Toronto was put under a heat alert on Tuesday, as the mercury rose steadily to a high of 33 C by the late afternoon. But the humidity made it feel even hotter.

The temperature was on a similar ascent in nearby cities including Vaughan, Brampton and Mississauga, which each hit 33 C by the end of the afternoon.

East of Toronto, the temperature hit 29 C in Pickering and Oshawa during the day.

The hot air was a factor across southern Ontario and in most parts it was feeling like 40 C, or higher, with the humidex factored in.

Also Tuesday, Environment Canada issued a humidex advisory, warning people to take precautions.

"This is day one of three days of humidex values into the 40s," said CBC meteorologist Johanna Wagstaffe earlier in the day.

"So it will feel like 43 C outside today, Wednesday and Thursday — temperatures alone soaring into the 30s. But with that humidity we're approaching dangerous levels and that's why Environment Canada has issued the advisory. Sort of a heads up to the public: stay indoors, seek shade, drink lots of water because this is not only uncomfortable but dangerous when we get these kinds of humidity levels," she said.

The heat alert issued by Toronto's chief medical officer advises residents to "call or visit family, friends and neighbours, especially isolated adults and seniors who are at greater risk of suffering from heat-related illness, to make sure they are cool and drinking plenty of fluids."

People are advised to drink plenty of fluids and to not to do too much strenuous work outside during the hot weather.

"Friday we're looking to break a cold front will finally move through. By Friday, temperatures back down to the mid-20s," Wagstaffe said.